Hepatitis C virus (HCV) is the most common blood-borne infection in the United States, with an overall prevalence of ~2%, and an estimated 200 million chronically infected people worldwide. A substantial body of evidence, including work from our laboratory, supports the concept that early events in the coordination and nature of multi-cellular immune responses are critical in determining whether the virus is cleared or whether persistence is established. However, despite the fact that approximately 40,000 pregnancies occur each year in HCV-infected women, little is known about the immunopathogenesis of HCV in this setting, in part because pregnant women with chronic HCV have hitherto been excluded from studies of immunity. For the first time, we have identified HCV-specific CD8+ T cells within the maternal-fetal interface that we hypothesize demonstrate versatile functional attributes that prevent transmission in the majority of cases. Furthermore, we present evidence that trophoblasts, specialized cells of the placenta that play important roles in embryo implantation and interaction with decidualized maternal uterus, can respond to a viral product of hepatitis C (known as a pathogen-associated molecular pattern or PAMP) by producing high levels of Type III IFNs (interferon lambda 3). These intriguing results corroborate the recent studies demonstrating genetic associations with single nucleotide polymorphisms that encode interferon lambda 3 and spontaneous recovery from HCV. We will also characterize how the HCV PAMP affects apoptosis signaling and differentiation of trophoblasts. In the third aim, building on strong preliminary data that ? T cells are significantly expanded within the placentas of HCV-positive mothers who do not transmit virus to their infants, we will characterize the phenotype, function, and restriction of these cells, implicated in other models as a first line of defense against viral infections. Thus, our proposal seeks to mechanistically understand the different cells and signals at the maternal-fetal interface that underpin transmission versus protection and cause detrimental effects within the placenta. Although focused on HCV infection, the results generated will have far-reaching implications for other viral pathogens that affect pregnancy.